What does a paroxysmal nocturnal hemoglobinuria (PNH) clone assay result indicate and how should a patient with a clone ≥1% be managed?

PNH Clone Assay: Clinical Significance and Management

What the Test Indicates

Flow cytometry detecting deficiency of GPI-anchored proteins (CD55 and CD59) on peripheral blood cells is the gold-standard diagnostic test for PNH, and any detectable clone ≥1% confirms the presence of a PNH population. 1

The assay evaluates:

• Granulocytes, monocytes, and red blood cells for absence of CD55 and CD59 expression 1
• Neutrophils for CD16b and CD66b deficiency 2
• Clone size as a percentage of affected cells, which directly correlates with clinical risk 3, 4

Clinical Significance by Clone Size

Patients with PNH clone >60% of granulocytes face significantly higher risk of thrombosis and hemolysis, making clone size a critical prognostic marker 3, 4:

• Clone ≥60%: High thrombotic risk, typically presents with classic hemolytic PNH 3
• Clone 10-60%: Variable clinical manifestations, may have subclinical hemolysis 1
• Clone 1-10%: Often associated with bone marrow failure syndromes (aplastic anemia, MDS), may be clinically silent 3, 1

When PNH Testing is Indicated

The following clinical scenarios warrant PNH clone testing:

• Unexplained intravascular hemolysis with negative direct antiglobulin test, elevated LDH, low haptoglobin, and hemoglobinuria 1, 5
• Venous thrombosis at unusual sites, particularly splanchnic veins (hepatic, portal, mesenteric); 9-19% of Budd-Chiari syndrome patients have underlying PNH 3, 1
• Cytopenias with bone marrow failure syndromes, especially hypoplastic MDS with normal cytogenetics or aplastic anemia 3, 1
• Young patients with unexplained cytopenias and hypoplastic marrow 1

Management Algorithm for Patients with Clone ≥1%

Initial Comprehensive Evaluation

All patients with confirmed PNH clone require immediate hematology consultation and comprehensive workup 1:

• Complete blood count with differential and reticulocyte count to assess degree of cytopenias and hemolysis 1
• Hemolysis markers: LDH, haptoglobin, indirect bilirubin, free hemoglobin 1, 2
• Renal function tests (hemoglobinuria causes renal damage) and coagulation profile 1
• Bone marrow aspiration and biopsy with cytogenetics to evaluate for concurrent aplastic anemia or MDS 3, 1

Risk Stratification Based on Clone Size and Clinical Features

For clones ≥60% with active hemolysis or thrombosis history:

• Immediate consideration for complement inhibitor therapy (eculizumab), which dramatically reduces hemolysis and thrombotic risk 5, 6
• Thromboprophylaxis evaluation: JAK2V617F mutation testing and comprehensive thrombophilia screening to exclude concurrent myeloproliferative neoplasms 3, 1

For clones 10-60%:

• Close monitoring with serial CBCs, LDH, and clinical assessment for hemolysis or thrombosis 1
• Anticoagulation consideration if any thrombotic event occurs, even with smaller clones 3

For clones 1-10% associated with bone marrow failure:

• Primary focus on treating underlying aplastic anemia or MDS 3, 1
• Serial flow cytometry monitoring every 6-12 months, as clone size may expand over time 3

Specific Therapeutic Interventions

Transfusion support:

• Packed red blood cells for symptomatic anemia, with all products irradiated and leukocyte-filtered when concurrent bone marrow failure exists 1

Anticoagulation:

• Lifelong anticoagulation is warranted for patients with any thrombotic event, given the high recurrence risk 3
• Primary prophylaxis with vitamin K antagonists reduces but does not eliminate thrombotic risk 6

Curative therapy:

• Allogeneic hematopoietic stem cell transplantation is the only cure and should be considered for severe bone marrow failure with PNH clone, particularly in young patients with matched donors 1
• HSCT is not first-line due to inherent morbidity and mortality, reserved for life-threatening or refractory disease 1

Critical Monitoring Requirements

Regular surveillance for all patients with PNH clone ≥1%:

• Thrombosis monitoring: Maintain high clinical suspicion for unusual site thrombosis, particularly Budd-Chiari syndrome 3, 4
• Hemolysis markers: Serial LDH, haptoglobin, and peripheral smear examination for schistocytes 4
• Clone size reassessment every 6-12 months by flow cytometry, as expansion indicates increased risk 3, 1
• Renal function monitoring due to chronic hemoglobinuria 1

Common Pitfalls to Avoid

• Do not dismiss small clones (1-10%) as clinically irrelevant—they may expand and are often associated with bone marrow failure requiring specific management 3, 1
• Do not delay anticoagulation in patients with thrombosis, even with small clone sizes, as PNH-associated thrombosis carries high mortality 3, 6
• Do not overlook concurrent hematologic disorders—PNH frequently coexists with aplastic anemia and MDS, requiring bone marrow evaluation 3, 1, 5
• Do not rely solely on Ham or sucrose lysis tests—flow cytometry is the definitive diagnostic standard 1, 2